Abstract: Method of producing steel slag free from disintegration completed through the modification of slag in molten state by use of a small amount of modifier while increasing the temperature of slag, wherein the chemical composition of the steel slag and modifier to be charged is analyzed beforehand, a small amount of boron-containing compounds and the first modifier consisting of silicate-containing minerals, etc. by which the basicity of the steel slag after treated settles in the range between 1.6 and 1.9 are mixed to the slag and, further, the exothermic and powdery second modifier consisting of one or two and more among aluminum, aluminum dross, magnesium, and magnesium slag is charged together with agitating gas by bubbling.